The subject matter of the present invention relates to well logging tools, and in particular, to a sonic transmitter for a sonic well logging tool.
Well logging tools are used in boreholes of oil wells for the purpose of determining if oil exists in a formation of the borehole. Some well logging tools are sonic in nature, in that they transmit a sonic pressure wave into the formation and receive the sonic pressure wave from the formation. The received sonic pressure wave is analyzed for the purpose of determining if oil exists in the formation. The sonic well logging tools include a transmitter for transmitting the sonic pressure wave into the formation. One such transmitter is discussed in an article entitled "Development of a Suspension PS Logging System's Seismic Source for Hard Ground", by Kenji Tanaka, Shingo Inoue, and Kimio Ogura, of OYO Corp, Japan (hereinafter referred to as the "Tanaka transmitter"). The Tanaka transmitter is used in subsoil logging for the construction industry, not in well logging. Furthermore, the Tanaka transmitter, being an air filled device, is capable of logging only shallow, low pressure boreholes. Furthermore, the Tanaka transmitter uses fixed solenoid coils to drive a metal hammer. The hammer impacts the ID of a thin walled cylindrical sleeve surrounding the sonde body, sending a pressure pulse out into the borehole fluid. This system has two disadvantages: the dipole radiation pattern from the struck sleeve is not symmetrical and, as a result, the Tanaka transmitter will produce a large, undesirable monopole component; the frequency response of the system is not controllable and is likely to be dominated by the resonant frequencies of the sleeve. Another such transmitter is discussed in a article entitled "A new method for shear-wave logging", by Choro Kitsunezaki, published in GEOPHYSICS dated October 1980; in an article entitled "Receivers and Sources in the Suspension Type Shear Wave Logging", by Choro Kitsunezaki, published in GEOPHYSICAL EXPLORATION dated 1983; and in U.S. Pat. No. 4,207,961 to Kitsunezaki (hereinafter referred to as the "Kitsunezaki transmitter"). The piston in the Kitsunezaki transmitter contains two coils, which are activated sequentially in time, rather than simultaneously. The Kitsunezaki transmitter uses one coil to drive the piston in one direction, the other coil to drive the piston in the opposite direction in order to prevent the magnetic field produced by the coils from opposing the permanent magnet's magnetic field and demagnetizing the permanent magnet. Further, the Kitsunezaki transmitter piston surrounds a centrally disposed permanent magnet, which presents three major disadvantages: since the Kitsunezaki magnet is disposed inboard of the coils, the magnet volume must be very small for a given package size thereby maintaining the flux in the magnet gap at a smaller level; the Kitsunezaki transmitter piston mass must be high and, as a result, given a piston area and a piston resonance frequency, Kitsunezaki must carry the mass of TWO driving plates and of the connecting rod which must join the two driving plates of the piston; since the Kitsunezaki piston is hollow, it carries a large, dead (acoustically non-radiating) internal fluid mass. The Kitsunezaki piston has two driving plates, each with an inwardly directed face, and a connecting rod passing through a close fitting hole in the magnet. Thus, the Kitsunezaki transmitter has a large internal area subject to fluid drag and squeeze film damping. It is likely that the Kitsunezaki transmitter will be overdamped.